(1) Field of the Invention
The present invention relates to a high-temperature fuel cell comprising a cell stack and gas supply/exhaust manifolds secured to the cell stack's side surfaces with gas-sealing sheets and insulators being interposed therebetween, and more particularly, to a structure of the insulators and manifolds.
(2) Description of the Related Art
A conventional fuel cell with external manifolds is of a structure shown in FIG. 1. A plurality of cells 72 are layered and sandwiched by end-plates 71a and 71b to form a cell stack 73, and a reactant gas manifold 74 is secured to each side surface of the cell stack 73. Since the manifold 74 is made of metal (stainless), a frame insulator 75 is placed between the cell stack 73 and manifold 74 for insulation. Further, porous ceramic felts 76 serving as gas-sealing sheets are placed between the manifold 74 and insulator 75, and insulator 75 and cell stack 73, respectively. The manifold 74 is pressed toward the cell stack 73 by an unillustrated pressing member to be secured to the cell stack 73 with the insulator 75 and felts 76 interposed therebetween.
As is shown in FIG. 2, the frame insulator 75 is a single component made of ceramics (e.g., high purity alumina) that remains stable when subject to an electrolyte, a fuel gas, and an oxidant gas at a temperature of about 650.degree. C., a fuel cell's operating temperature.
However, the frame insulator 75 thus formed expands as the fuel cell's temperature increases during the operation, and accordingly the felts 76 have gaps with the manifold 74 and cell stack 73, causing the reactant gas to leak.
In addition, the cell stack 73, manifold 74, and insulator 75 have different coefficients of thermal expansion. Thus, a change in the fuel cell's temperature caused by the increase or decrease in temperature or an abrupt change in load within the fuel cell results in an irregular load onto the insulator 75 from the manifold 74 in a direction to the cell stack 73. Such an irregular load causes the reactant gas to leak, or worse, the insulator 75 to crack; a considerable amount of the reactant gas will leak in the latter case.
Further, since the insulator 75 is the single ceramic component, forming the same requires quite a many man-days, and retains only a low yield rate. Moreover, when a large-scale fuel cell is assembled, the insulator 75 is enlarged accordingly and baked in a corresponding size furnace, which increases the manufacturing costs undesirably.